This invention relates to freeze indicators. More particularly, it relates to freeze indicators which may be adjusted to provide precise information to the user that a product has been exposed to a certain low temperature, usually near the freezing point of water.
Freeze indicators which utilize the expansion characteristic of water to break a frangible ampule are well known in the art; see for example, Smith U.S. Pat No. 3,545,400. Once the device shown in the Smith '400 patent is exposed to temperatures below the freezing point of water, the water freezes into ice and expands causing the ampule to break. After the ice has formed and after the surrounding temperature has returned to a point above the melt point of the ice, the water is absorbed on a dye loaded pad, thus giving an indication that the device has gone through a freeze stage and back through a thaw stage.
Problems have arisen in giving an accurate indication of the passage of the device through the normal freezing point of water, i.e. 0.degree. C., due to the undercooling effect of water which will permit water to remain in its liquid state substantially below its normal freezing point as for example, as low as -16.degree. C. This problem has been partially overcome by the addition of certain nucleating agents to the water. An example of this is shown in British Patent No. 1,245,135, issued to Scheller. Scheller discloses the technique of adding powdered glass to an ammonium chloride solution to avoid undercooling.
U S. Pat. Nos. 3,956,153 and 3,980,581, issued respectively to Chadha and Godsey, disclose the use of nucleating agents having space groups substantially similar to space groups of thermally responsive materials used in disposable thermometers. Chadha '153, discloses the use of anthraquinone as a regenerative nucleating agent.
It is known that over a period of time a nucleating agent will become "poisoned". Not wishing to be bound by theory this poisoning effect is believed to result either from contamination by the medium into which it is incorporated or by some as yet unexplained change with time in the crystal structure of the surface of the nucleating agent. The solution to this "poisoning" problem which Chadha proposed was to incorporate into the thermally responsive material a nucleating agent which is slightly soluble in the thermally responsive material at a concentration in excess of the solubility. The result is that with each remelt and nucleation cycle a fresh surface of nucleating agent is presented which effectively nucleates the thermally responsive medium. Of course where the preferred nucleating agent for a system is insoluble, the approach of Chada cannot be utilized.
An improved freeze indicator has been disclosed in U S. Patent No. 4,191,125 to Johnson. That '125 patent discloses a device comprising a water filled frangible ampule, a nucleating agent and a surfactant. Suitable nucleating agents which are disclosed include cupric sulfide, ferrous sulfide, zinc metal, molybdenum sulfide, tungsten sulfide, beryllium aluminum silicate and silver iodide, all of which are substantially insoluble in water. These insoluble nucleating agents are susceptible to the poisoning effect discussed above.
More recently, an improved indicator of the type described by Johnson has been patented. See co pending U.S. application Ser. No. 07/72,335 to Larsson, now U.S. Pat. No. 5,111,768. The improvement of Larsson comprises utilizing an insoluble metal compound as the nucleating agent together with a soluble salt of the metal in the metal compound. Additionally, Larsson discloses utilizing an absorbent material to obscure the ampule from view. While this aspect of the Larsson freeze indicator is preferred by some customers for the device, placing the absorbent material into the packaging containing the ampule is labor intensive. What is required is a simplified method of obscuring the ampule from view in a manner which does not require separate handling of the package to put the absorbent material in place.